A method of the above-described type is disclosed in published German Patent Document DE 197 57 197 and U.S. Pat. No. 5,198,390, which will be mentioned in greater detail below.
Although applicable to any number of micromechanical devices and structures, particularly switches and light modulators for displays, the basic underlying problem definition relating to the present invention is explained with reference to a micromechanical oscillating mirror device which can be manufactured using the technology of silicon surface micromechanics.
Micromechanical oscillating mirror devices are used, for example, in integrated optics to switch over the path of light rays between single optical wave guides (optical fibers) or those situated in an array. Such oscillating mirror devices are known in various design variants, and have a controllable drive—integrated into the mirror or situated next to the mirror on the same chip—which generates the tilting motions of the mirror surface, and therewith the optical switching procedures.
These tilting motions, in which the movable, micromechanically exposed part of the oscillating mirror device, that is, the island region, whose upper side forms the mirror surface, is able to execute torsional vibrations which have an amplitude such that a part of the island region reaches into the area of the silicon substrate layer that has been etched free, assume that there is a deep-running free space under the island region, according to the size of the desired tilt angle. The area etched bare naturally has to have at least the lateral dimensions of the island region.
Large deflections perpendicular to the chip surface, as are required for oscillating mirror devices or other optical components in micromechanics (optical MEMS), are currently not able to be produced in surface micromechanics, since the usual sacrificial layers and etching techniques used permit only the motion of the structures exposed by a few micrometers perpendicular to the surface, which corresponds to a tilt angle of only a few degrees, which is insufficient for many applications.
Optical components having large vertical deflections, e.g., oscillating mirror devices, are, as is described, for example, in published German Patent Document DE 197 57 197 cited above, at this time, therefore, mostly achieved in that the mirror structures are structured from the wafer front and are disengaged from the backside of the carrier substrates using the methods of volume (bulk) micromechanics. In this context, SOI material is generally used and it is necessary to etch through the silicon substrate layer starting from the backside, with the aid of anisotropic wet etching, up to the upper silicon layer (or oxide layer), while the lateral exposing of the island region, before or after wet etching, is done by a dry etching method.
A front side deep etching method, using which, micromechanical structures are exposed laterally and vertically, is described in U.S. Pat. No. 5,198,390 that was mentioned at the outset. In that document, the so-called SCREAM process (single crystal reactive etching and metallization process) starts from a uniform monocrystalline silicon wafer, i.e., it is not able to be subdivided into separate functional layers and sacrificial layers, in which all structuring steps and deep etching steps are carried out uninterruptedly with the aid of reactive ion etching (RIE). In this context, first of all, the etching mask is structured by RIE etching, subsequently the corresponding trench structures are generated in the wafer by RIE deep etching, then, by selective masking of the side walls of the trenches, the structure to be exposed is specified, and finally the substrate region below the selected (island) structure is exposed by complete etching undercutting using RIE. The SCREAM process stands out, on the one hand, by great process uniformity, and on the other hand, by the etching undercutting takes place at a low etching rate typical of the process, which has a negative effect, particularly in the case of wide selected (island) structures. Especially problematic with respect to applications particularly for oscillating mirror devices is the fact that the unprotected underside of the selected (island) structure, in response to etching undercutting of the region of the silicon substrate lying below it, is considerably attacked as well.